This invention relates to an electronic hearing device for stimulating the auditory nerve of a human patient by way of implanted electrodes. Such devices are necessary for people who have malfunctioning or non-functioning inner ear mechanisms.
There are some prior art devices which receive sound signals, convert said sound signals to electrical signals, divide those signals into channels based on frequency separation, and then apply the outputs of those channels to separate electrodes along the basilar membrane within the cochlea of a human patient. Many, if not most of those devices, involve the conversion of sound signals into digital electronic signals which are applied to the electrodes. Some systems, however, such as U.S. Pat. Nos. 3,751,605 and 4,400,590 apply analog signals representative of sounds to the auditory nerve.
In the normal ear, auditory nerve fibers are distributed along the cochlear basilar membrane. Because of the mechanical tuning of this membrane and its associates structures, nerve fibers that innervate near the basal end are sensitive to high frequencies while the more apical fibers are sensitive to lower frequencies. A multichannel device for auditory nerve stimulation that uses a plurality of filters to produce a plurality of component signals having different frequency ranges should distribute these component signals to electrodes in a relationship that is consistent with the frequency analysis preformed by the normal cochlea. Assume, for instance, that four channels of filter processing are used to produce four component signals named S1, S2, S3 and S4 with S1 having the lowest range of frequencies, S2 the next lowest, S3 the next lowest and S4 the highest range. These signals should be distributed to corresponding electrodes E1, E2, E3, E4, where E1 is placed in the cochlea more apical than E2, E2 more apical than E3 and E3 more apical than E4. Ideally, the position of E1 in the cochlea would correspond to the position of auditory neurons that are most sensitive to the center frequency of the channel that produces S1. Likewise for the other channels and their corresponding electrode positions.
It has been discovered that the amount of amplification necessary for each of the frequency channels is not necessarily uniform. In the Michelson Pat. No. 4,400,590, the gains of the respective channels are empirically adjusted to meet the particular needs of the user. This is an extremely difficult task since the user has no basis of comparison by which to tell the proper adjustment. Also, the adjustment is difficult to make with regard to discriminating speech sounds, which vary widely in frequency content.